Conventionally, an intake air amount control system for an internal combustion engine, which is described in Patent Literature 1, is known as the intake air amount control system of the above-mentioned kind. The engine has an electromagnetic valve mechanism and a valve lift sensor provided for each cylinder, which changes the valve-closing timing relative to the valve-opening timing of each cylinder as desired. The valve lift sensor detects the amount of valve lift of each cylinder. As described hereinafter, in this intake air amount control system, to control the idling speed, the valve-closing timing of each intake valve is controlled via the electromagnetic valve mechanism, whereby the intake air amount is controlled.
More specifically, the target intake air amount for feedforward control is calculated according to the target engine speed, and further, an average value of valve-closing times of the intake valves of all the cylinders is calculated based on the valve-closing time of each intake valve detected by the valve lift sensor. Then, the maximum value of the absolute values of the differences between the average value and the valve-closing times of the cylinders is calculated. Further, according to the maximum value of the absolute values of the differences, a gain for feedback control is calculated. The target intake air amount for feedback control is calculated according to the gain, and the valve-closing timing of each intake valve is calculated based on the two target intake air amounts for feedback control and feedforward control and so forth and the valve-closing timing of the intake valve is controlled by the calculated valve-closing timing. The valve-closing timing of each intake valve is controlled as described above, whereby the idling speed is controlled such that it converges to a target engine speed.
According to the conventional intake air amount control system, based on the valve-closing time of the intake valve detected by the valve lift sensor, the gain for feedback control is calculated, and based on the calculated gain, the target intake air amount for feedback control is calculated. Further, the valve-closing timing of the intake valve is only controlled based on the target intake air amount for feedback control. Therefore, it is impossible to compensate for dead time of the control system, such as operation delay of the electromagnetic valve mechanism, which degrades the convergence of the idling speed to the target engine speed, and hence controllability is low. What is more, it is impossible to compensate for variation in dynamic characteristics and aging of the control system, such as variation in the dynamic characteristics and aging of the electromagnetic valve mechanism, and drifts of the output from the valve lift sensor caused by aging thereof, which degrades the robustness of the control system. As a consequence, in the conventional intake air amount control system, due to the low controllability and low robustness of the system described above, the intake air amount control becomes unstable, which causes rotational variation, and hence there is a fear that during the idling speed control, engine stall occurs, and the amount of exhaust emissions increases due to the degraded fuel combustion. Further, when the above-described method of controlling the intake air amount control is applied to the intake air amount control in a normal operation load region, torque variation and rotational variation increase and the degree of degradation of the combustion also increases, so that drivability and exhaust emissions are further degraded. This problem becomes conspicuous in a high load region or during lean operation (during execution of EGR).
Further, a control system described in Patent Literature 2 is known as the control system of the above-mentioned kind. The control system controls the air-fuel ratio of an internal combustion engine as a plant, and is comprised of a LAF sensor, an oxygen concentration sensor, a state predictor, an identifier, and a sliding mode controller. The LAF sensor and the oxygen concentration sensor detect parameters indicative of the oxygen concentration of exhaust gases in an exhaust passage of the engine, and are arranged in the exhaust passage at respective locations from the upstream side. In this control system, as the controlled object model, there is employed a discrete time system model to which is inputted the difference (hereinafter referred to as “the LAF difference”) between the detected signal value of the LAF sensor and a reference value and from which is outputted the difference (hereinafter referred to as “the O2 difference”) between the detected signal value of the oxygen concentration sensor and a predetermined target value, as the controlled object motel, whereby a control input for controlling the air-fuel ratio is calculated as follows:
The state predictor calculates a predicted value of the O2 difference with a predetermined prediction algorithm based on the controlled object model, and the identifier identifies the model parameters of the controlled object model by a sequential least-squares method. Further, the sliding mode controller calculates the control input based on the predicted value of the O2 difference and the identified values of the model parameters with a sliding mode control algorithm such that time-series data of the O2 difference as a state variable converges to a value of 0. As a consequence, the air-fuel ratio is controlled such that the detected signal value of the oxygen concentration sensor converges to a predetermined target value. With the sliding mode control algorithm, the control input is calculated as the sum of an equivalent control input, an adaptive law input, and a reaching law input. The adaptive law input is for compensating for a modeling error of the controlled object model.
According to the conventional control system, with the sliding mode control algorithm, the modeling error of the controlled object model is compensated for by the adaptive law input. Therefore, when there occurs a steady-state deviation (offset) between the predicted value of the O2 difference and the actual value of the same, i.e. between the predicted value of the output from the plant and the detected value of the same, the steady-state deviation cannot be compensated for, so that there is a possibility that the steady-state deviation remains. Although such a steady-state deviation does not present problems in the above-mentioned air-fuel ratio control, in control demanding higher control accuracy (e.g. control for positioning an actuator), the control system may fail to achieve the demanded control accuracy due to the influence of the steady-state deviation.
The present invention has been made so as to solve the above problems, and a first object thereof is to provide an intake air amount control system for an internal combustion engine, which is capable of ensuring high robustness and improving controllability in air fuel ratio amount control, to thereby improve drivability and reduce exhaust emissions.
A second object of the invention is to provide a control system which is capable of compensating for a steady-state deviation between a predicted value of an output from a plant and a detected value of the same, to thereby enhance control accuracy.
[Patent Literature 1]
Japanese Laid-Open Patent Publication (Kokai) No. 2001-140661 (pages 5 and 6, FIGS. 6 to 18) [Patent Literature 2]
Japanese Laid-Open Patent Publication (Kokai) No. 2000-179385 (pages 11 to 19, FIG. 3).